Interferometers, as used in spectroscopy, are instruments that require a high degree of precision in manufacture. They must be assembled and maintained with accuracies of a small fraction of a wavelength. While such tolerances are routine in optomechanics today, it is expensive to build an instrument that requires such tight tolerance, especially if one or more components must move while holding this accuracy, as is the case with a scanning Michelson or Fabry-Perot interferometer. In the Michelson interferometer (or any two-beam interferometer) the delay between photons is changed by scanning a moving mirror in one leg of the interferometer. In correlation interferometry, the delay between two correlated photons is measured directly with very fast electronics.
Other concepts relating to correlation interferometer spectroscopy and interferometric spectroscopy in general are disclosed in, U.S. Pat. No. 6,504,614 to Messerschmidt et al. for Interferometer Spectrometer with Reduced Alignment Sensitivity; U.S. Pat. No. 7,324,210 to De Groot for Scanning Interferometry for Thin Film Thickness and Surface Measurements; U.S. Pat. No. 7,321,431 to De Groot for Method and System for Analyzing Low-Coherence Interferometry Signals for Information about Thin Film Structures; U.S. Pat. No. 7,315,382 to De Groot for Interferometry Method for Ellipsometry, Reflectometry, and Scatterometry Measurements, including Characterization of Thin Film; U.S. Pat. No. 7,304,745 to Towers et al. for Phase Measuring Method and Apparatus for Multi-Frequency Interferometry; U.S. Pat. No. 7,280,224 to Hill et al. for Interferometry Systems and Methods of Using Interferometry Systems; U.S. Pat. No. 7,280,223 to Hill et al. for Interferometry Systems and Methods of Using Interferometry Systems; U.S. Pat. No. 7,251,041 to Hill for Spatial Filtering in Interferometry; U.S. Pat. No. 7,139,081 to De Groot et al for Interferometry Method for Ellipsometry, Refectometry, and Scatterometry Measurements, including Characterization of Thin Film Structures; US 2007/0103694 to Kato for Interferometry System; US 2001/0042831 to Wood et al. for Photon Detector; US 2007/0041011 to Hayden et al. for Fast Time-Correlated Multi-Element Photon Detector and Method; US 2004/0178348 to Wainer et al. for Pixelated Photon Detector.
One way to overcome the intricacies of interferometers is to make the precision components “solid state” or “monolithic.” If this can be achieved, an instrument with such components will be more rugged. For example, to achieve this, Fabry-Perot interferometers sometimes have novel scanning mechanisms. One scanning or tuning approach is to vary the refractive index of the material in the interferometer cavity. This has been done both with liquid crystal materials, and by changing the pressure of a gas in the cavity. Another way to overcome this dilemma is to eliminate the use of optics entirely. Correlation phenomena happen routinely, as light waves of sufficient coherence interact with each other. No special optical instruments are really needed to detect these effects. In any interference spectroscopy, the delay between two photon paths must be measured or varied. Table 1 lists the measurements that can be made, and how those measurements can be used or applied.
TABLE 1MEASUREDDERIVEDPARAMETERPARAMETERAPPLICATIONSFringe PositionMean phaseLength standardsdifferenceLength comparisonMachine controlRefractometryvelocity of lightPhase variationsMicrotopographyOptical testingFringe VisibilitySpectrum sourceProfile ofsymmetrical linesSpatial distributionStellar diametersat sourceFull IntensitySpectrum of SourceDirect interferenceDistributionspectroscopy(position andFourier spectroscopyvisibility)Spatial distributionOptical transferat sourcefunction hologramGiven this background, there exists a need for a correlation interferometric spectroscopy system that eliminates the need for precision optical components all-together in the attainment of spectra through a process of interference.